Just moments after Aaron Alexis opened fire in Building 197 at the Washington Navy Yard on Sept. 16, agents from the Naval Criminal Investigation Service were on the scene.

At a press conference Wednesday, law enforcement officials from NCIS, the Federal Bureau of Investigation and other agencies released new details in the ongoing investigation into the Navy Yard shooting.

An NCIS agent reporting for duty at Naval Sea Systems Command headquarters was the first to respond, as he saw employees streaming out of the building minutes after Alexis, the government subcontractor and former Navy sailor, fired the first shots at 8:16 a.m.

“[The agent’s] car was in the parking lot right across from the building, and he immediately grabbed ... his vest and his helmet out of the car” before entering the building to engage the shooter, NCIS Washington Field Office Special Agent-in-Charge Mike Monroe told Navy Times.

Meanwhile at NCIS headquarters in Quantico, Va., Special Agent Darrell Gilliard, the deputy assistant director for criminal investigations and operations, received a computer alert from the Washington field office that the Navy Yard was dealing with an active shooter situation. His management team set up a crisis action center in the agency’s Multiple Threat Alert Center, a 24/7 operation that monitors criminal, counterintelligence and terrorism threats around the world.

Gilliard said his team focused on relaying information to and from the field office, researching any leads and generally chasing down requested information.

Facts shifted throughout the day. Early on there was some indication that Alexis didn’t act alone, Monroe said, based on security camera footage and information gathered from witnesses.

“That information was immediately broadcast to all the responding law enforcement, to attempt to identify them,” he explained. “As the morning unfolded, we were able to identify and fully interview those witnesses, and we found that those individuals were not suspects, but were part of the response or were command members in the area leaving the building.”

Meanwhile, investigators interviewed each evacuated employee.

“We started screening individuals for witness or evidentiary testimony or information,” Monroe said. “As they were leaving, we were both interviewing individuals and obtaining names for further interviews.”

NCIS also worked with the FBI to get the shooter’s background. Alexis, a former sailor, had recently arrived in Washington to work as a contractor with a private information technology company.

He started work Sept. 9. On Sept. 13, there was a “routine performance-related issue addressed to him,” but Valerie Parlave, the assistant director in charge of the FBI Washington field office, said there was no indication that workplace issues motivated the attack.

This month, NCIS agents are conducting field briefs with sailors and Marines, encouraging them to identify and report potentially unstable co-workers, from unusual behavior to actual threats.

He added that NCIS had never received information on Alexis as a potential threat.

“It’s very important that the work force continue to work together in identifying individuals that have stressors in their lives, to try to help their shipmates,” he said. “Because obviously, there are a lot of individuals that could be provided care, that would help them as they move through their day-to-day lives.”

Going forward, officials said, NCIS is working jointly with the FBI, the Washington Metropolitan Police Department and other agencies to complete the investigation.